The assembly of aircraft components, and especially larger components, presents various challenges. In particular the assembly may have to be carried out to very narrow tolerances but also at reasonable speed and as economically as possible. An example where such issues arise is in the assembly of a wing skin to rib feet to form a wing-box of an aircraft. In a conventional process, the wing skin is manufactured in a desired shape and is then brought into position against the sub components (rib and spars, for example) of the underlying structure of the wing-box to which the wing skin is to be secured. That underlying structure has outwardly projecting rib feet on which respective parts of the inner surface of the wing skin are required to rest so that fasteners can be inserted through the wing skin and the rib feet to secure them together. The external profile of the wing skin is important in terms of the aerodynamic performance of the aircraft and a strong connection between the wing skin and the rib feet is also important in terms of the structural strength of the wing-box.
Some tolerances have to be allowed for during manufacture and consequently when the wing skin is brought in an unstressed state into position against the rib feet, it is commonly found that, whilst some rib feet are in contact with the wing skin, others are spaced from it. To eliminate the spacings, there are two approaches that are adopted. A first approach is simply to deform the wing skin by the small amount necessary to bring the other rib feet into contact with the wing skin. An approach of that kind, however, results in a distortion of the external profile of the wing skin, which may adversely affect the aerodynamic performance of the wing, and in the introduction of additional internal stresses into the wing-box, which may adversely affect the structural strength of the wing. To avoid such problems, a second approach involving altering the dimensions of the underlying structure or the wing skin may be adopted. There are various ways in which that may be done, including fettling the rib feet, adding material to the wing skin or applying a shim to the rib feet.
There are also other parts in an aircraft, especially, but not exclusively, in a wing-box, where parts have to be assembled to close tolerances and liquid or solid shims are used.
Commonly, where a liquid shim is used, one of three different liquid shimming techniques is employed. In a first technique a solid shim of appropriate thickness is bonded to a part and a liquid shim (a substance that is applied as a liquid but cures to a solid) is applied over the solid shim. A second technique uses a liquid shim only and is used mainly for smaller gaps. A third technique involves first applying a liquid shim to a part and then applying a solid shim over the liquid shim. After the shim has been applied the parts to be joined are brought together. In order for the correct thickness of liquid shim to be present, it is important that the part to which the shim is applied is brought into position adjacent to the part it is to be joined to before the liquid shim is used. That introduces an unwelcome time constraint, which is particularly significant when many areas require shimming in a single assembly, for example where a wing skin is joined to a multiplicity of rib feet.
It is an object of the invention to provide a method of preparing a first part for assembly to form an aircraft component, in which method the problem referred to above is at least mitigated.